The isoprene rule is the foundation of terpene classification. All terpenes are built from isoprene, a five-carbon molecule (C5H8), assembled in multiples. A monoterpene contains two isoprene units (C10H16). A sesquiterpene contains three (C15H24). A diterpene contains four (C20H32). This building block architecture creates a predictable relationship between molecular size and physical properties: larger terpenes are less volatile, slower to evaporate, and often more viscous at room temperature.
Monoterpenes: the volatile front
Monoterpenes (C10) are the lightest and most volatile terpene class. They evaporate quickly, which is why they hit your nose first. Limonene delivers that immediate citrus burst. Alpha-pinene gives pine forests their distinctive smell. Linalool creates the floral opening of lavender. Because monoterpenes evaporate at low temperatures, they're the most heat-sensitive compounds in any extract: steam distillation and high-temperature vaporization degrade them fastest. In GC/MS analysis, monoterpenes appear early in the chromatogram precisely because they elute first due to their lower molecular weight.
Sesquiterpenes: the character and complexity
Sesquiterpenes (C15) are heavier, less volatile, and tend to define a terpene profile's depth and complexity rather than its opening impression. Beta-caryophyllene, the dominant sesquiterpene in cannabis, delivers spicy, peppery notes and uniquely binds CB2 receptors in the endocannabinoid system. Myrcene, often classified as a monoterpene in some systems but technically a sesquiterpene in others, gives cannabis and hops their earthy, musky character. Humulene produces the woody, herbal notes associated with traditional indica profiles. Sesquiterpenes survive heat and extraction better than monoterpenes, which is why they tend to dominate the profiles of cured and processed cannabis products.
Beta-caryophyllene does something no other terpene can: it binds directly to CB2 receptors in the human endocannabinoid system, making it eligible for classification as both a terpene and a dietary cannabinoid. It's the reason black pepper and cannabis share overlapping aromatic and pharmacological character.
Diterpenes and beyond: the stable backbone
Diterpenes (C20) and triterpenes (C30) are too heavy to be volatile; you can't smell them in the traditional sense. They show up as structural and protective compounds in plant waxes, resins, and hormones. Phytol, a diterpene, is part of the chlorophyll molecule. Retinol (Vitamin A) is a diterpene derivative. These are not the compounds you'll find in terpene blends or aromatic applications, but understanding they exist on the same molecular continuum helps explain why the plant produces such a varied chemical library from the same isoprene starting point.
- Monoterpenes (C10): Limonene, pinene, linalool, terpineol: highly volatile, heat-sensitive, fragrance top notes
- Sesquiterpenes (C15): Caryophyllene, myrcene, humulene, bisabolol: moderate volatility, characterful, more stable
- Diterpenes (C20): Phytol, taxol derivatives: non-volatile, structural plant compounds